Modern automotive vehicles include a fuel tank and an evaporative emission control system that collects fuel vapors generated in the fuel tank. The evaporative emission control system includes a vapor collection canister that collects and stores fuel vapors. The canister, which is typically a carbon canister that contains an activated charcoal mixture, collects fuel vapors which accumulate during refueling of the vehicle or from increases in fuel temperature. The evaporative emission control system also includes a purge valve placed between an intake manifold of an engine of the vehicle and the canister. The purge valve is opened by an engine control unit in order to purge the canister. The collected fuel vapors are drawn into the intake manifold from the canister for combustion within a combustion chamber of the engine.
Vehicle diagnostic systems monitor certain performance and functionality characteristics of the evaporative emission control system. For example, the vehicle diagnostic system may determine if a leak exists in the system. In one such system, a vacuum regulator sensor unit draws a vacuum on the evaporative emission control system and senses whether a loss of vacuum occurs within a specified period of time. In systems that utilize positive pressurization, the evaporative emission control system is pressurized to a set pressure. A sensor determines whether the pressure remains constant over a certain period of time.